“This discovery conjures up an image of a Campbell garrison fleeing from the castle when under attack, dropping and losing one of their most precious items, or maybe the seal had once been hidden within a wall niche and long forgotten.”

The Campbells and the MacDonalds fought over Dunyvaig in the early 17th century, with a series of bombardments of the castle until the Campbells finally prevailed.

Sir John took ownership of Islay in 1615.

Mr Regan said the seal could have been lost in a later raid in 1646 when Alasdair MacColla, a descendent of the MacDonalds, reclaimed the castle.

He said: “Alasdair MacColla, a descendent of the MacDonalds, retook the castle and installed his elderly father, Colla Ciotach, to defend it.

“The castle was immediately besieged again.

“We may have Colla Ciotach’s hasty defences in the form of turf walls built above the already ruined stone walls of Dunyvaig, before he was forced to surrender in 1647 and then hanged from the castle walls.”

The excavation at Dunyvaig is being undertaken by charity Islay Heritage in partnership with the University of Reading.

A team of around 40 experts, including leading archaeologists, geophysicists, scientists and environmentalists, are almost at the end of an initial three-week dig.

Professor Steven Mithen, chairperson of Islay Heritage and director of the Dunyvaig Project added: “Coming towards the end of the dig, after the team had worked so hard to move huge amounts of turf and rubble, this has been a thrilling discovery.

“We have found a piece of Islay’s past and Scottish history. We can’t wait to start digging again in 2019.”

Islay Heritage hopes to raise further funds to allow excavation work to continue at Dunyvaig over the next five years.

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